Modern heating and cooling systems are increasingly required to control indoor humidity levels in addition to the sensible air temperature. One reason for this is the desire to provide ever higher levels of occupant comfort. Improving indoor air quality through dehumidification is important for health reasons as well, because high humidity is associated with mold and mildew and other unpleasant and hazardous conditions.
In the cooling mode, air conditioners typically provide dehumidification only as a byproduct of the cooling process, with 20-30% of the total cooling capacity usually being apportioned to latent cooling (dehumidification) and the balance to sensible cooling (the measurable reduction in air temperature across the cooling coil). In most applications, this proportionate level of dehumidification is adequate during those periods when the cooling system is operating on a nearly continuous basis. However, during periods of low space cooling demand, the cooling system will not operate long enough to remove the amount of moisture required to control indoor humidity. There are also applications where dehumidification is required during periods when sensible cooling is not needed at all. Thus, there are situations where an air conditioning system must provide dehumidification independent of sensible cooling.
One means of providing a dehumidification mode in a cooling system is referred to as a “reheat” process. In a reheat process, the supply air is reheated to a comfortable level after being cooled for adequate dehumidification. There are several known techniques to perform the reheating process, such as electric resistance heaters, desuperheating or condensing heat exchangers connected to the cooling refrigerant system, and heat exchangers connected to a boiler. The heat source in the preferred methods is some form of waste heat generated in the system as a result of the cooling process. This greatly improves the energy efficiency of the dehumidification process, as no new energy is consumed for reheat.